Re: [PATCH v2] ARM: Don't use complete() during __cpu_die

From: Mark Rutland
Date: Tue Feb 10 2015 - 10:15:02 EST

On Tue, Feb 10, 2015 at 01:24:08AM +0000, Stephen Boyd wrote:
> On 02/05/15 08:11, Russell King - ARM Linux wrote:
> > On Thu, Feb 05, 2015 at 06:29:18AM -0800, Paul E. McKenney wrote:
> >> Works for me, assuming no hidden uses of RCU in the IPI code. ;-)
> > Sigh... I kind'a new it wouldn't be this simple. The gic code which
> > actually raises the IPI takes a raw spinlock, so it's not going to be
> > this simple - there's a small theoretical window where we have taken
> > this lock, written the register to send the IPI, and then dropped the
> > lock - the update to the lock to release it could get lost if the
> > CPU power is quickly cut at that point.
> Hm.. at first glance it would seem like a similar problem exists with
> the completion variable. But it seems that we rely on the call to
> complete() fom the dying CPU to synchronize with wait_for_completion()
> on the killing CPU via the completion's wait.lock.
> void complete(struct completion *x)
> {
> unsigned long flags;
> spin_lock_irqsave(&x->wait.lock, flags);
> x->done++;
> __wake_up_locked(&x->wait, TASK_NORMAL, 1);
> spin_unlock_irqrestore(&x->wait.lock, flags);
> }
> and
> static inline long __sched
> do_wait_for_common(struct completion *x,
> long (*action)(long), long timeout, int state)
> ...
> spin_unlock_irq(&x->wait.lock);
> timeout = action(timeout);
> spin_lock_irq(&x->wait.lock);
> so the power can't really be cut until the killing CPU sees the lock
> released either explicitly via the second cache flush in cpu_die() or
> implicitly via hardware.

That sounds about right, though surely cache flush is irrelevant w.r.t.
publishing of the unlock? The dsb(ishst) in the unlock path will ensure
that the write is visibile prior to the second flush_cache_louis().

That said, we _do_ need to flush the cache prior to the CPU being
killed, or we can lose any (shared) dirty cache lines the CPU owns. In
the presence of dirty cacheline migration we need to be sure the CPU to
be killed doesn't acquire any lines prior to being killed (i.e. its
caches need to be off and flushed). Given that I don't think it's
feasible to perform an IPI.

I think we need to move the synchronisation down into the
cpu_ops::{cpu_die,cpu_kill} implementations, so that we can have the
dying CPU signal readiness after it has disabled and flushed its caches.

If the CPU can kill itself and we can query the state of the CPU, then
the dying CPU needs to do nothing, and cpu_kill can just poll until it
is dead. If the CPU needs to be killed from another CPU, it can update a
(cacheline-padded) percpu variable that cpu_kill can poll (cleaning
before each read).

> Maybe we can do the same thing here by using a
> spinlock for synchronization between the IPI handler and the dying CPU?
> So lock/unlock around the IPI sending from the dying CPU and then do a
> lock/unlock on the killing CPU before continuing.
> It would be nice if we didn't have to do anything at all though so
> perhaps we can make it a nop on configs where there isn't a big little
> switcher. Yeah it's some ugly coupling between these two pieces of code,
> but I'm not sure how we can do better.

I'm missing something here. What does the switcher have to do with this?

> > Also, we _do_ need the second cache flush in place to ensure that the
> > unlock is seen to other CPUs.
> >
> > We could work around that by taking and releasing the lock in the IPI
> > processing function... but this is starting to look less attractive
> > as the lock is private to irq-gic.c.
> With Daniel Thompson's NMI fiq patches at least the lock would almost
> always be gone, except for the bL switcher users. Another solution might
> be to put a hotplug lock around the bL switcher code and then skip
> taking the lock in gic_raise_softirq() if the IPI is our special hotplug
> one. Conditional locking is pretty ugly though, so perhaps this isn't
> such a great idea.

There are also SMP platforms without a GIC (e.g. hip04) that would need
similar modifications to their interrupt controller drivers, which is
going to be painful.

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